Process for obtaining treated coal and silica from coal containing fly ash

ABSTRACT

A process for treating mixtures of solid coal and fly ash containing metallic oxides, silicon dioxide and sulfur compounds in order to produce treated coal and substantially pure silicon dioxide comprising (1) reacting a mixture of the coal and fly ash with hydrogen fluoride in water to produce a liquid stream comprising silicon fluoride and metal fluorides and a solids stream comprising unreacted coal and sulfur compounds; (2) reacting the sulfur compounds with metallic nitrates dissolved in water to form an aqueous solution of nitrate, metallic and sulfur ions; (3) separating the aqueous solution of nitrate, sulfur and metallic ions from the solid coal; (4) washing the previously treated solid coal with water; (5) reacting the silicon fluorides and metal fluorides with metallic nitrates in an aqueous mixture to form solid silicon dioxide; and separating the solid silicon dioxide from the aqueous mixture.

BACKGROUND OF THE INVENTION

The present invention relates to a process for removing mineral impurities present in coal containing particulate fly ash, including metal oxides such as Al₂O₃, SiO₂, Fe₂O₃, as well as sulfur, calcium and other oxides normally found in coal ash. The process results in a treated and cleaner coal product, while at the same time allowing for the separation and recovery of relatively pure (commercial grade) silicon dioxide (SiO₂) as a valuable byproduct.

The commercial use of untreated coal as fuel, particularly coal containing sulfur and fly ash, has long been known to result in potentially unacceptable levels of air pollution as well as high maintenance costs for industrial plants relying on coal as the primary hydrocarbon fuel source. The presence of contaminants such as nitrogen and sulfur-based compounds in coal fly ash has two significant drawbacks. First, the presence of fly ash tends to reduce the potential heat value of the coal, making it less thermally efficient in plant processes. Second, the fly ash can cause significant environmental and/or operating problems in industrial applications, such as direct coal-fired turbine plants used to generate electricity. As a result, over the years the need to reduce and/or eliminate contaminants in carbon-based fuels has resulted in a reduction in domestic use of coal containing fly ash because of increased production costs and strict environmental concerns.

In addition, the installation and operating costs of power generation plants using coal with fly ash are typically higher due to the need for pollution control equipment to scrub and/or eliminate exhaust gases in order to comply with increasingly strict federal and state environmental control regulations. Coal-powered plants also typically suffer higher maintenance costs associated with cleaning of plant equipment contaminated by coatings on surfaces exposed to coal combustion.

Although some progress has been made over the years in increasing the efficiency of coal-fired processes, a significant need still exists to develop cleaner and more efficient coal for use in combined cycle power generation systems in order to increase the amount of power produced per unit of CO₂ emitted. Unfortunately, the coal suitable for turbine plants cannot contain more than very small amounts of particulate fly ash in order to operate efficiently. Numerous R&D programs have been undertaken during the past decade to reduce the fly ash content, with most programs focusing on chemical treatment to produce an ultra-low ash coal. Typically, those methods involved some form of caustic and/or acid treatment of the coal raw materials in an effort to extract the ash constituents as soluble species and separate the dissolved ash constituents from the coal fuel. Examples of such processes have met with only limited success and include, for example, caustic treatments that form soluble sodium alumino-silicates or hydrofluoric acid based treatments that form fluoro-complexes.

One disadvantage of these earlier systems is that they typically involve high operating temperatures, high acidity, and considerable amounts of water that cannot be discharged into the environment without secondary treatment because the effluent contains soluble organic and inorganic species that pollute other natural resources. The large amount of water needed also puts pressure on the resources necessary to treat water downstream of any production facility, rendering most processes impractical from a cost standpoint. Many in-force pollution control regulations require processes to approach a near zero-discharge of the species removed from the coal and the recovery of the water used for the coal treatment.

Another clear deficiency of currently available coal treatment systems is their inability to recover potentially valuable silicon dioxide present in the fly ash. Currently, the worldwide production of silica is about 40,000 tons/year with the demand increasing steadily and approaching 75,000 tons/year. The cost of commercial-grade silica has also increased due to increasing demand for alternative solar energy sources and because existing commercial SiO₂ processes are cost prohibitive.

The present invention provides a significant advancement in the art by creating two independent and commercially valuable products from the same raw material coal feed, namely the production of treated coal using the hydrofluoric acid/nitrate process described herein, and the production of commercial grade silica (silicon dioxide) for possible use in solar energy panels and the semiconductor industry. The treated coal produced from processes described herein offers particular advantages for coal fired turbine plants that need to meet environmental control standards to produce electricity in a combined cycle power plant. Likewise, the recovery of commercial grade silicon dioxide from the same raw material provides additional cost benefits and commercial advantages over conventional coal treatment processes.

The use of treated coal per se as a potential energy source is known. However, to date it has not been feasible to economically produce such coal using raw materials containing unwanted components resident in the fly ash particulates, particularly sulfur or undesirable oxides (e.g., aluminum, iron, calcium and silicon), while at the same time producing commercial grade (relatively pure) silica as a byproduct. Normally, the ash from the coal is converted to slag/fly ash and then collected and sold as a low value product for other uses, such as cement production, structural fill material or an asphalt paving constituent. The ash content of coal varies from location to location, but typically falls in the range of about 5-7 weight percent in the U.S. In 2001, U.S. electric utilities produced over 70 million tons of fly ash, of which about 25 million tons could be used as an ingredient for cement production and related industries. Thus, large amounts of fly ash are not being used and ultimately must be discarded as a waste product.

Potential uses for treated coal, apart from power generation, are well known and include production of heavy fuel oil, graphite and carbon fibers. The coal itself also has non-fuel uses, e.g., as a raw material for manufacturing high purity carbon-based products, including electrodes for the aluminum industry. Likewise, pure silica can be used in the manufacture of a wide variety of products, such as silicon chips and solar cells.

BRIEF DESCRIPTION OF THE INVENTION

An exemplary process according to the invention includes the following process steps for treating mixtures of solid coal and fly ash containing metallic oxides, silicon dioxide and sulfur compounds in order to produce treated coal and substantially pure silicon dioxide: (1) reacting a mixture of coal and fly ash with hydrogen fluoride in water to produce a liquid stream comprising silicon fluoride and metal fluorides and a solids stream comprising unreacted coal and sulfur compounds (e.g., metal sulfides); (2) reacting the sulfur compounds with metallic nitrates dissolved in water to form an aqueous solution of nitrate, metallic and sulfur ions; (3) separating the aqueous solution of nitrate, sulfur and metallic ions from the solid, initially treated coal; (4) washing the previously treated coal with water; (5) reacting the silicon fluoride and metal fluorides with metallic nitrates in an aqueous mixture to form silicon dioxide as a solid component; and separating the silicon dioxide product from the aqueous mixture.

The invention also comprises dissolving, and thereafter separating, metallic oxides present in the fly ash component of the coal being treated (such as Al₂O₃ and Fe₂O₃), as well as treating and removing substantially all of the sulfur compounds, e.g., iron sulfide and aluminum sulfide. In exemplary embodiments, the process has the capability to reduce the fly ash content of the resulting treated coal down significantly, preferably to levels at or below 0.01 weight percent. In addition, in order to recover hydrogen fluoride for use in the initial fluoride reaction with metallic oxides in the fly ash, the invention uses a high temperature reaction with metallic fluoride components generated during the earlier reaction steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block flow diagram depicting exemplary process steps for producing treated coal and substantially pure silica in accordance with the present invention; and

FIG. 2 is a detailed process flow diagram depicting the major pieces of process equipment and associated process streams and control parameters for an exemplary process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

As noted above, the present invention provides for an efficient separation of metal and inorganic constituents of coal fly ash while also providing the almost complete recovery of commercial-grade silica present in the fly ash. The method reduces the unwanted fly ash constituents (such as metal oxides, sulfur and silicon dioxide) to an impurity level that easily meets current environmental control regulations without requiring complex and expensive emission control equipment, particularly in coal-fired turbine plants, power stations and the like. At the same time, the method provides an economical process for recovering essentially pure silica as a valuable byproduct.

An exemplary process according to the invention isolates metal “impurities” present in the fly ash by converting the components to soluble mineral oxides that can be removed as waste from the system. The process also reforms, and then segregates, the silica component for removal as a separate product. As a result, very little silica remains in the treated coal end product or in any remaining fly ash. As noted above, in prior art processes, even small quantities of silica in the coal fly ash could result in a commercial disadvantage due to the extremely abrasive qualities of the silica and inherent reduction in thermal efficiency associated with non hydrocarbon components in coal used as fuel. The process thus has the advantage of removing a high proportion of the silica present in the fly ash. In addition, substantially all of the hydrogen fluoride that reacts with the silicon present in the fly ash is later recovered and recycled for use in the process.

With particular reference to FIG. 1, the basic process flow steps for producing treated coal for and byproduct silica according to the invention are shown generally in order to achieve three basic process objectives, namely the initial treatment of the fly ash constituents containing silicon dioxide and metal oxides using hydrogen fluoride; the subsequent removal of residual sulfur and nitrate compounds to produce an treated coal product; and the regeneration of essentially pure silica into solid form after being separated from the initial fly ash components that remain in liquid.

As FIG. 1 illustrates, the initial “dirty” coal feed 10 containing fly ash typically includes solid coal with silicon dioxide, metal oxides such as aluminum and iron oxide, and relatively small, but environmentally significant, amounts of sulfur compounds. In one embodiment, the initial fly ash reaction occurs using a hydrogen fluoride feed 11 in a mixed batch reaction step as shown at 12 to generate SiF₄, as well as metal fluorides (such as aluminum and iron fluorides), that dissolve in the water present in the process. The resulting solid stream includes initially treated coal stream 13 which is then separated as a wet solid along with a lesser amount of solid unreacted sulfur compounds and residual metal oxides. This preliminarily clean coal undergoes a second basic reaction step using nitric acid shown being added via nitric acid line 14 which allows for the removal and eventual separation of residual sulfur and nitrate compounds from the coal and fly ash in the nitrate treatment and sulfur removal steps 15. The end result includes treated coal product 16.

As FIG. 1 also illustrates, the products of the initial reaction of fly ash compounds with hydrogen fluoride results in the formation of SiF₄ and various metal fluorides soluble in the water present in the initial reaction mixture. An aqueous stream containing dissolved metal fluoride and silicon fluoride compounds 17 can be combined with the reaction products from the nitrate treatment and sulfur removal steps at 15 which include metal nitrates as noted above. The silicon fluoride and nitrates react at SiO₂ formation step 19 to form solid silicon oxide which can be separated and removed from the system in essentially “pure” form as solid SiO₂ 20 a. The remaining metal fluoride compounds and nitrate ions remaining in solution are then separated and treated in downstream operations, ultimately forming metal oxides as disposable waste and regenerating hydrogen fluoride as shown by recycle and regeneration line 20 b for recycle and use in the initial reaction sequence.

Referring now to the exemplary and more detailed process and equipment flow diagram of FIG. 2, the raw material feed to the system shown generally as STEP 1 includes raw coal having a relatively low moisture content and containing a fly ash constituent comprising metal oxides such as Al₂O₃ and Fe₂O₃. The fly ash also typically includes smaller, but commercially significant, amounts of SiO₂ and residual amounts of FeS₂ and CaO that ultimately must be removed to produce treated coal. The mixed coal and fly ash stream at 23 is fed into stirred reactor 21, together with an aqueous hydrogen fluoride stream 24 (nominally about 1% by weight HF in water) to undergo an initial reaction (typically at about 150° F. with a reaction pressure near atmospheric), as exemplified by the following general formula:

SiO₂+4HF→SiF₄+2H₂O

The batch reaction taking place in reactor 21 occurs over a period of about 2-3 hours and can be controlled using a water (or steam) jacket 22 surrounding stirred reactor 21 as the reaction components are being agitated. The resulting “coal slurry,” which includes fluoride reaction products dissolved in water and entrained solids (shown by the double coal slurry process line 25) is transferred from stirred reactor 21 to vacuum drum filter 26 shown as STEP 2. The drum filter removes substantially all of the liquid component. The liquid fraction from the drum filter includes dissolved, but unreacted hydrogen fluoride, as well as fluoride compounds formed in the initial reaction dissolved in water, including silicon fluoride (SiF₄), aluminum and iron fluoride, and potentially other small quantities of metallic fluorides depending on the species of metallic oxides present in the initial coal and fly ash feed.

Drum filter 26 separates out the initially “clean” coal product along with other solid compounds still remaining in the mixture which do not dissolve in water, including iron sulfides, calcium oxide and like solid compounds, using a conventional knife edge 28 to remove the solids as shown. The separated mixture 29 of wet coal product and other minor solids is transferred to nitrate reactor 30 as depicted in STEP 3 which operates to remove the residual amounts of sulfur, calcium and inorganic compounds from the coal feed by virtue of a reaction with a stream of metallic nitrates and water (see feed 32 to nitrate reactor 30). An exemplary reaction of the liquid nitrates/water stream with the metal sulfide components being fed to nitrate reactor 30 is shown by the following general formula:

FeS₂+14FE(NO₃)₃+8H₂O→2SO₄ ²⁻+16H⁺+15Fe²⁺+42NO³⁻

The liquid stream 27 leaving vacuum drum filter 26 at STEP 2, which contains a small amount of unreacted hydrogen fluoride and the mix of metallic fluorides and silicon fluorides mentioned above, is fed to downstream mixed reactor 40 as shown at STEP 6 as explained in more detail below.

Meanwhile, the reaction products of nitrate reactor 30, which include the original coal feed (a solids component) and the products of the above reaction (now in solution), leave reactor 30 via nitrate reactor discharge line 31. The entire liquid/solids stream containing the reaction products of reactor 30 move through a second vacuum drum filter 33 shown at STEP 4 which removes the entrained liquid 34 containing the various dissolved compounds while separating out the moist coal product 36, again using a conventional knife edge 35. Coal product 36 then moves through a wash station 37 at STEP 5 where any final, residual amounts of waste compounds soluble in water are removed (labeled “water wash” 39), resulting in a clean coal product 38 containing less that 0.1 weight % dry ash and preferably less than 0.01 weight %.

An exemplary reaction of the liquid components fed to mixed reactor 40 at STEP 6 is shown below.

SiF₄+2(Al₂Fe)(NO₃)₃+2H₂O→SiO_(2(s))+2(Al, Fe)F₂ ⁺+4H⁺+6NO₃ ⁻

As the above formula indicates, the silicon fluoride components created during the initial reaction at STEP 1 react with metallic nitrates in solution to form silicon dioxide as the principal reaction product, as well as metal fluorides (e.g., aluminum and iron) and nitric acid, all of which remain dissolved in the water fraction. Significantly, the silicon dioxide is now solid in form. The combined liquid/solid slurry containing the SiO₂ leaves mixed reactor 40 through bottom discharge line 41 and passes through a third vacuum drum filter 42 in STEP 7 which removes substantially pure silicon dioxide product 45 as the solids fraction. Again, the moist, but substantially pure, SiO₂ solid product can be removed as shown using a conventional knife edge 44 with the separated liquid fraction containing aqueous metal fluorides and nitric acid leaving via drum filter discharge 43 for further treatment and eventual recycle.

The liquid fraction from filter drum 42 containing metallic fluorides and dissolved nitric acid is transferred from drum filter 42 through a plurality of separation chambers 46 shown operating in series as 46 a, 46 b and 46 c, which remove a substantial fraction of the entrained water using, for example, reverse osmosis. The separated liquid stream 47 is processed further to separate out the nitric acid/water from the metallic fluoride components using distillation column 48 as shown in STEP 8. The distillation process includes a conventional reboiler 53 using steam 55, with the vapor 54 being fed from the reboiler back into distillation column 48. The system also includes condenser 51 with overhead vapor line 50 and condensate recycle 52 as shown. The nitric acid and water taken off column 48 provide the liquid feed 56 to mixed reactor 57 (discussed below).

The metallic fluoride compounds resident in the feed to the distillation column at STEP 8 are removed from the bottom of the column as a bottoms feed 49 to high temperature hydrolizer 60 in STEP 9 (operating at about 750° F.). In order to maintain the proper temperature control of the reaction components, hydrolizer 60 includes a steam jacket 61. The reaction inside hydrolizer 60 follows the general formula shown below:

As the above general formula indicates, hydrolizer 60 produces make-up hydrogen fluoride overhead as shown by overhead vapor line 63, which is then quenched with water at quench station 64 in STEP 10 and fed to the initial metal oxide stirred reactor 21 described above. Make-up HF can also be added as shown at HF line 65. The solid bottoms product from hydrolizer 60 includes residual metallic oxide compounds, a portion of which can be returned as residual feed 59 to reactor 57 in STEP 11 to react with recycle nitric acid to form the metal nitrates used in the reaction taking place in reactor 30 as described above. The remaining metallic oxide components from hydrolizer 60 can be discarded as waste 62.

The substantially pure silicon dioxide product 45 removed in STEP 7 of FIG. 2 can be used to produce various end products including, for example, semi-conductor components, soda-lime glass (typically found in drinking glasses), whiteware ceramics such as earthenware, stoneware, food additives (primarily as a flow agent in powdered foods), high-temperature thermal protection fabrics and cosmetics due to its light-diffusing properties. Due to its relatively pure form, end product 45 can also be used as a starting material in reactions involving various basic metal oxides, e.g., sodium oxide, potassium oxide, lead(II) oxide, zinc oxide or mixtures of oxides forming silicates and glasses, including borosilicate glass and lead glass.

SiO₂ product 45 can also serve as a raw material in the production of polydimethylsiloxane (PDMS) having the following general formula:

PDMS belongs to a group of polymeric organosilicon compounds commonly referred to as “silicones” known for their unusual rheological (flow) properties. The applications for PDMS range from contact lenses and medical devices to elastomers in shampoos, caulking, lubricating oils and heat resistant tiles.

The SiO₂ can also be used in the production of oil resistant silicon rubber compounds having the general formula:

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A method for treating mixtures of solid coal and fly ash containing metallic oxides, silicon dioxide and sulfur compounds to produce reacted or treated coal and substantially pure silicon dioxide, comprising the steps of: reacting said mixture of coal and fly ash with hydrogen fluoride in water to produce a liquid stream comprising silicon fluoride and metal fluorides, and a solids stream comprising unreacted coal and sulfur compounds; reacting said sulfur compounds with metallic nitrates dissolved in water to form an aqueous solution of nitrate, metallic and sulfur ions; separating said aqueous solution of nitrate, sulfur and metallic ions from said solid coal; washing said coal with water; reacting said silicon fluoride and metal fluorides with metallic nitrates in an aqueous mixture to form solid silicon dioxide; and separating said solid silicon dioxide from said aqueous mixture.
 2. A method according to claim 1, wherein said metallic oxides comprise Al₂O₃ and Fe₂O₃.
 3. A method according to claim 1, wherein said sulfur compounds comprise iron and aluminum sulfides.
 4. A method according to claim 1, wherein the amount of fly ash remaining in said treated coal is less than about 0.01 wt. %.
 5. A method according to claim 1, wherein said reaction of silicon fluoride with metallic nitrates to form silicon dioxide occurs according to the following general formula: SiF₄+2(Al₂Fe)(NO₃)₃+2H₂O→SiO_(2(s))+2(Al, Fe)F₂ ⁺+4H⁺+6NO₃ ⁻
 6. A method according to claim 1, wherein said reaction of silicon dioxide with hydrogen fluoride to produce silicon fluoride occurs according to the following general formula: SiO₂+4HF→SiF₄+2H₂O
 7. A method according to claim 3, wherein iron sulfide reacts with iron nitrate according to the following general formula: FeS₂+14Fe(NO₃₎ ₃+8H₂O→2SO₄ ²⁻+16H⁺+15Fe²⁺+42NO³⁻
 8. A method according to claim 1, wherein said reaction of fly ash containing silicon dioxide occurs at a temperature of about 150° F. and at a pressure of about atmospheric.
 9. A method according to claim 1, further comprising the step of forming metallic nitrates for use in said reaction with sulfur compounds by reacting nitric acid (HNO₃) with metallic fluoride compounds having the general formula (Al,Fe)F₂OH in water.
 10. A method according to claim 1, further comprising the step of recovering metallic oxides of aluminum and iron and regenerating hydrogen fluoride using a high temperature reaction according to the following general formula:


11. A method according to claim 10, wherein said reaction occurs at a temperature of about 750° F. 